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Best roof stackup for Houston, TX

user-1112697046 | Posted in GBA Pro Help on

This is on an original single story ranch style house with no way to ins under the deck.

here is the proposed stackup.

2×6 rafters, original 1950s 1×6 plank deck, either vap open or vap closed peal and stick, one 3″ and one 2″ layer foil faced ISO, top layer taped with a tape that can accommodate ISO shrinkage (suggestions ?), 1.5″ vent channel,

Standing seam metal (Kynar high SRI on top and bare galvalum on bottom .06 emissivity).

The big question is vapor open or vapor closed peal and stick and what to do to limit any odor wafting through the ship lap plank decking

also am having trouble deciding on vapor open or closed peal and stick on walls with 3″ foil ISO plu rainscreen under ship-lap siding
any rec commendations for a drain plane behind the ISO and plywood sheathing or is that a waste of time

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Michael.

    With foil faced polyiso, it doesn't matter if your underlayment is vapor open or closed. Your sheathing will need to be able to dry inward should it get wet and it doesn't sound like you have any materials beneath the sheathing that will prevent that from happening. This answers the same question about your walls as well.

    I have heard of people getting odors inside their homes when installing underlayments over board sheathing. Two common recommendations are an interior air barrier, for example airtight drywall on the ceiling, and slight presurization of the home.

    Some builders use a drainable housewrap between the wall sheathing and continuous exterior rigid foam for a bit of drainage potential should water get back there. This slightly, but relatively insignificantly reduces the performance. Other builders don't feel it is necessary. I'm not sure that there is a fully adhered (peel-and-stick) draining WRB on the market at this point, but perhaps one will be available soon, so keep your eyes peeled for that.

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